1. Field of the Invention
The invention relates in general to a touch sensing technique, and more particularly, to techniques for reinforcing immunity from noise for a touch sensing device.
2. Description of the Related Art
Operating interfaces of recent electronic products are becoming increasingly user-friendly and intuitive. For example, through a touch screen, a user can directly interact with applications as well as input messages/texts/patterns with fingers or a stylus, thus eliminating complications associated with other input devices such as a keyboard or buttons. In practice, a touch screen usually comprises a touch panel and a display provided at the back of the touch panel. According to a touch position on the touch panel and a currently displayed image on the display, an electronic device determines an intention of the touch to execute corresponding operations. Existing touch sensing techniques are roughly categorized into resistive, capacitive, electromagnetic, ultrasonic and optic types; among which, the mutual-capacitance touch sensing technique prevails in supporting multi-touch, and has been widely adopted in many products during the recent years.
A mutual-capacitive touch panel comprises a plurality of electrodes made of a transparent conductive material, with the electrodes being alternately arranged on the entire touch panel. Referring to FIG. 1A showing a top view of a touch panel of the prior art, a plurality of electrodes 12 in an X direction are driving electrodes, and a plurality of electrodes 14 in a Y direction are sensing electrodes. The two types of different electrodes form a matrix pattern comprising a plurality of sensing units. As shown in FIG. 1A, each of the row driving electrodes 12 is connected to a driver 16, and each of the column sensing electrodes 14 is connected to a receiver 18. In general, the drivers 16 sequentially send out driving signals that are then continuously received by the receivers 18.
FIG. 1B is a detail view of FIG. 1A, and FIG. 10 is a front view of FIG. 1B. A sensing unit 20 is defined by one driving electrode 12 and one sensing electrode 14, as shown in FIG. 1B. In this prior art, the driving electrode 12 and the sensing electrode 14 are provided on two different planes that are parallel to each other and vertical, i.e., perpendicular, to a Z direction. Since the two types of electrodes are designed to have different levels, a predetermined number of power lines 32 are present between the two types of electrodes. When a finger 30 approaches the sensing unit 20, a part of the power lines 32 between the driving electrode 12 and the sensing electrode 14 are attracted by the finger 30 due to a ground-like effect, such that the mutual capacitance between the driving electrode 12 and the sensing electrode 14 decreases to result in a mutual capacitance variation, which is then reflected by an output signal from the receiver 18 connected to the sensing electrode 14. According to a position of the receiver 18 and a position of a driver 16 sending out a driving signal at the time of the touch, a subsequent circuit determines an X/Y coordinate of the touch point.
It should be noted that power lines that are affected by the finger 30 are mainly distributed in areas 22A and 22B in FIG. 1B, i.e., two edge areas of an intersection of the driving electrode 12 and the sensing electrode 14 in the top view. Because of shielding effects, power lines below the intersection of the sensing electrode 14 and the driving electrode 12 are not largely affected by the finger 30. In other words, the above mutual capacitance variation mainly arises from variations in the power lines of the areas 22A and 22B.
In the prior art shown in FIG. 1A, the driving electrodes 12 and the sensing electrodes 14 are long strip-shaped electrodes having a same width. However, electrodes of current touch panels are not limited to such pattern; FIGS. 2A and 3A show two other current electrode patterns. In FIG. 2A, the driving electrodes 12 are wider than the sensing electrodes 14. However, for the sensing units in FIG. 2A, corresponding areas of power lines that are affected by user touch are similarly limited to two edge areas at an intersection of the driving electrode 12 and the sensing electrode 14, as areas 23A and 23B shown in FIG. 2B.
In FIG. 3A, the driving electrodes 12 and the sensing electrodes 14 are rhombuses. FIG. 3B is a detail view of FIG. 3A. Neighboring driving electrodes 12 of a same row are connected to one another through a bridge in an X direction; neighboring sensing electrodes 14 of a same column are connected to one another through a bridge in a Y direction. In this example, the sensing unit 20 is defined by two driving electrodes 12 and two sensing electrodes 14. For the sensing units 20 in FIG. 3B, mainly affected power lines by a user touch are distributed in areas 24A to 24D. As shown from FIG. 3B, the areas 24A to 24D are substantially neighboring ranges of diagonals lines of the sensing unit 20.
In order to provide touch sensing accuracy of a certain extent, areas of the sensing units 20 in FIGS. 1B, 2B and 3B are usually approximately the same, e.g., 5 mm by 5 mm. For a single sensing unit, immunity from noise interference gets larger as a mutual capacitance variation caused by a user gets larger, facilitating a subsequent circuit to determine a position of a touch point more accurately. In certain electronic systems supporting multi-touch, to enlarge the mutual capacitance variation for resisting against noise induced by multiple fingers, a potential between the driving electrode 12 and the sensing electrode 14 increases to 10 to 20 volts. Apart from high power consumption, such approach suffers from a drawback that associated circuits are required to withstand high voltages. As a result, hardware costs of such a touch screen are drastically increased.